1. Field of the Invention
The present invention relates to a display device including a display unit having a light emitting element and a pixel circuit for each of pixels, and a drive unit driving the pixel circuit, and to a method for driving the same. The present invention also relates to an electronic device having the display device.
2. Description of the Related Art
In recent years, in the field of a display device for displaying an image, a display device using, as a light emitting element of a pixel, an optical element of a current driving type whose light emission luminance changes according to the value of a flowing current, for example, an organic EL (Electro Luminance) element is developed and is being commercialized.
An organic EL element is a spontaneous light emitting element different from a liquid crystal element or the like. Consequently, in a display device using an organic EL element (organic EL display device), a light source (backlight) is unnecessary. As compared with a liquid crystal display device necessitating a light source, visibility of an image is higher, power consumption is lower, and response of the element is faster.
As driving methods of the organic EL display device, as in the liquid crystal display device, there are a simple (passive) matrix method and an active matrix method. The simple (passive) matrix method has, although the structure is simple, a disadvantage in that a large-sized high-resolution display device is difficult to be realized. Consequently, at present, the active matrix method is actively developed. In the active matrix method, current flowing in a light emitting element disposed for each pixel is controlled by an active element (generally, TFT (Thin Film Transistor)) provided in a drive circuit arranged for each of the light emitting elements.
Generally, a current-voltage (I-V) characteristic of the organic EL element deteriorates with time (time-dependent degradation). In a pixel circuit for current-driving an organic EL element, when the I-V characteristic of the organic EL element changes with time, the voltage dividing ratio between the organic EL element and a drive transistor connected in series with the organic EL element changes, so that voltage Vgs between the gate and the source of a drive transistor also changes. As a result, the value of current flowing in the drive transistor changes, so that the value of current flowing in the organic EL element also changes, and the light emission luminance also changes according to the current value.
There is a case that a threshold voltage Vth of the drive transistor and mobility μ change with time, or differ among pixel circuits due to variations in manufacturing processes. In the case where the threshold voltage Vth of the drive transistor and mobility μ differ among pixel circuits, the value of current flowing in the drive transistor varies among pixel circuits. Consequently, even when the same voltage is applied to the gate of the drive transistor, the light emission luminance of the organic EL element varies, and uniformity of a screen deteriorates.
A display device is developed, which has a function of compensating fluctuations in the I-V characteristic of an organic EL element and a function of correcting fluctuations in a threshold voltage Vth and mobility μ of a drive transistor in order to maintain the light emission luminance of the organic EL element without being influenced by variations with time in the I-V characteristic of the organic EL element and variations with time in the threshold voltage Vth of the drive transistor and the mobility μ (see, for example, Japanese Unexamined Patent Application Publication Nos. 2007-171827, 2007-108381, 2007-133283, and 2007-133284).
FIG. 15 illustrates an example of a schematic configuration of a display device in related art. A display device 100 illustrated in FIG. 15 has a display unit 110 in which a plurality of pixels 120 are disposed in a matrix, and a drive unit (a horizontal drive circuit 130, a write scan circuit 140, and a power source scan circuit 150) for driving each of the pixels 120.
Each pixel 120 includes a pixel 120R for red, a pixel 120G for green, and a pixel 120B for blue. As illustrated in FIG. 16, each of the pixels 120R, 120G, and 120B includes an organic EL element 121 (organic EL elements 121R, 121G, and 121B) and a pixel circuit 122 connected to the organic EL element 121. The pixel circuit 122 includes a transistor Tws for sampling, a retention capacitor Cs, and a transistor TDr for driving, and has a circuit configuration of 2Tr1C. A gate line WSL led from the write scan circuit 140 is formed so as to extend in the row direction and is connected to the gate of the transistor Tws. A drain line DSL led from the power source scan circuit 150 is also formed so as to extend in the row direction, and is connected to the drain of the transistor TDr. A signal line DTL led from the horizontal drive circuit 130 is formed so as to extend in the column direction, and is connected to the drain of the transistor Tws. The source of the transistor Tws is connected to the gate of the transistor TDr for driving and one end of the retention capacitor Cs. The source of the transistor TDr and the other end of the retention capacitor Cs are connected to the anode of the organic EL element 121R, 121G, or 121B (hereinbelow, simply referred to as the organic EL element 121R or the like). The cathode of the organic EL element 121R or the like is connected to a cathode line CTL.
FIG. 17 illustrates an example of various waveforms in the display device 100 illustrated in FIG. 15. FIG. 17 illustrates a state where two kinds of voltages (Von and Voff (<Von)) are applied to the gate line WSL, two kinds of voltages (Vcc and Vini (<Vthe1+Vca)) are applied to the drain line DSL, and two kinds of voltages (Vsig and Vofs) are applied to the signal line DTL. Vthe1 denotes a threshold voltage of the organic EL element 121R or the like, and Vca denotes a cathode voltage of the organic EL element 121R or the like. Further, FIG. 17 illustrates a state where the gate voltage Vg and the source voltage Vs of the transistor TDr change momentarily in accordance with application of the voltages to the gate line WSL, the drain line DSL and the signal line DTL.
Vth Correction Preparation Period
First, Vth correction is prepared. Concretely, the power source scan circuit 150 decreases the voltage of the drain line DSL from Vcc to Vini (T1). The source voltage Vs decreases to Vini, and light of the organic EL element 121R or the like goes out. At this time, the gate voltage Vg also decreases due to coupling via the retention capacitor Cs. Next, while the voltage of the signal line DTL is Vofs, the write scan circuit 140 increases the voltage of the gate line WSL from Voff to Von (T2). As a result, the transistor Tws is turned on, and the gate voltage Vg of the transistor TDr decreases to Vofs.
First Vth Correction Period
Next, Vth is corrected. Concretely, while the voltage of the signal line DTL is Vofs, the power source scan circuit 150 increases the voltage of the drain line DSL from Vini to Vcc (T3). Current Ids flows between the drain and source of the transistor TDr, so that the retention capacitor Cs and an element capacitor (not illustrated) such as the organic EL element 121R or the like are charged, and the source voltage Vs rises. After lapse of a predetermined period, the write scan circuit 140 decreases the voltage of the gate line WSL from Von to Voff (T4). The transistor Tws is turned off, the gate of the transistor TDr floats, and correction of Vth is temporarily stopped.
First Vth Correction Stop Period
In a period in which Vth correction stops, the voltage of the signal line DTL is sampled in another row (pixel) different from a row (pixel) subjected to the Vth correction. In the case where the Vth correction is insufficient, that is, in the case where a potential difference Vgs between the gate and the source of the transistor TDr is larger than threshold voltage Vth of the transistor TDr, also in the Vth correction stop period, in the row (pixel) subjected to the Vth correction, current Ids flows between the drain and source of the transistor TDr, the source voltage Vs rises, and the gate voltage Vg also rises by the coupling via the retention capacitor Cs. Since reverse bias is applied to the organic EL element 121R or the like, the organic EL element 121R or the like does not emit light.
Second Vth Correction Period
After completion of the Vth correction stop period, Vth is corrected again. Concretely, when the voltage of the signal line DTL is Vofs and Vth correction is possible, the write scan circuit 140 increases the voltage of the gate line WSL from Voff to Von (T5) and connects the gate of the transistor TDr to the signal line DTL. In the case where the source voltage Vs is lower than Vofs−Vth (in the case where the Vth correction has not been completed), the current Ids flows between the drain and source of the transistor TDr until the transistor TDr cuts off (until the voltage difference Vgs becomes Vth). As a result, the retention capacitor Cs is charged to Vth, and the potential difference Vgs becomes Vth. After that, before the horizontal drive circuit 130 switches the voltage of the signal line DTL from Vofs to Vsig, the write scan circuit 140 decreases the voltage of the gate line WSL from Von to Voff (T6). The gate of the transistor TDr floats so that the potential difference Vgs may be maintained at Vth irrespective of the magnitude of the voltage of the signal line DTL. By setting the potential difference Vgs to Vth as described above, also in the case where the threshold voltage Vth of the transistor TDr varies among the pixel circuits 122, light emission luminance of the organic EL elements 121R or the like may be prevented from varying.
Second Vth Correction Stop Period
After that, in the Vth correction stop period, the horizontal drive circuit 130 switches the voltage of the signal line DTL from Vofs to Vsig.
Write and μ Correction Period
After completion of the Vth correction stop period, writing and μ correction are performed. Concretely, while the voltage of the signal line DTL is Vsig, the write scan circuit 140 increases the voltage of the gate line WSL from Voff to Von (T7) and connects the gate of the transistor TDr to the signal line DTL. As a result, the voltage of the gate of the transistor TDr becomes Vsig. The voltage of the anode of the organic EL element 121R or the like is still smaller than threshold voltage Vel of the organic EL element 121R or the like at this stage, and the organic EL element 121R or the like cuts off. Consequently, the current Ids flows to an element capacitor (not illustrated) of the organic EL element 121R or the like, and the element capacitor is charged. The source voltage Vs rises only by ΔV, and the potential difference Vgs becomes Vsig−Vofs+Vth−ΔV. In such a manner, μ correction is performed at the same time with the writing. The larger the mobility μ of the transistor TDr is, the larger ΔV becomes. Therefore, by decreasing the potential difference Vgs only by ΔV before light emission, the variations in the mobility μ per pixel may be eliminated.
Light Emission
Finally, the write scan circuit 140 decreases the voltage of the gate line WSL from Von to Voff (T8). The gate of the transistor TDr floats, the current Ids flows between the drain and source of the transistor TDr, and the source voltage Vs rises. As a result, the organic EL element 121R or the like emits light with desired luminance.